hello elda, the site provides no certificate chain to the issuer of the certificate which will prompt firefox to shown the error message. please see www.sslshopper.com/ssl-certificate-not-trusted-error.html for background on the issue & troubleshooting advice.

Note that Firefox automatically stores intermediate certificates that servers send in the Certificate Manager for future usage. So you may not seeing an error if you have visited a website in the past that did send this intermediate certificate. If you have never visited a website that did use that same intermediate certificate then Firefox hasn't saved it and you will get an untrusted error.